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Hubble Space Telescope Observation Plans


HST's Planned WF/PC Observations of the May Ring Plane Crossing

During the May 1995 crossing of the Earth through Saturn's ring plane, the Hubble Space Telescope will monitor the changing brightness of the rings with the WF/PC2. Amanda Bosh of Lowell Observatory leads the HST investigation of the May ring-plane crossing.

Her group will obtain WF/PC2 images of Saturn and its rings for 7 orbits centered on the predicted time of the ring-plane crossing. Imaging for 7 orbits allows for uncertainty in the exact time of ring- plane crossing due to the unknown magnitude of the precession of Saturn's ring-plane pole (+/-2 hours). These exposures (20 to 30 minutes per orbit) will be obtained with a methane filter in order to minimize the contribution of scattered light from Saturn to background noise. Frames with shorter exposures will be obtained each orbit to allow for the removal of cosmic rays and photometric calibration using Saturn's satellites.

A measurement of the brightness of the rings in their edge-on configuration, combined with photometric properties of the rings derived from calibration observations will allow astronomers to determine the vertical thickness of the rings. During the last ring-plane crossing (1980) the ring thickness was measured to be approximately 1.1 km by Andre Brahic and others. However, a large uncertainty remains in this value for the rings' thickness due to a lack of suitable calibration targets and the relatively crude detectors then available. With the excellent seeing of the HST and the technological advances incorporated into WF/PC2, Bosh's team should be able to improve the determination of this value by more than a factor of 20. Knowledge of the thickness of the rings (a collisional many-body system) is crucial for several aspects of dynamic models of ring systems: their evolution, particle size distribution, reaction to perturbations, and more.

The time of the ring-plane crossing (the moment of minimum light from the rings) will help researchers determine the precession rate of Saturn's pole. Recent analyses (based on stellar occultation data, including one occultation observed in 1991 with the High Speed Photometer aboard the HST) allow for a wide range of values for the precession rate. The HST observations of the ring-plane crossings may offer the best opportunity for determining this value.

Results from the May 1995 ring-plane crossing will aid observers of the later crossings by providing a more refined determination of the times of these crossings and a measurement of the residual brightness of the rings. Later observations can be designed to optimize the data received in response to information from this first ring-plane crossing.

blue_dot.gif Four New Saturn Satellites Discovered by Hubble! (July 26, 1995).
blue_dot.gif Hubble Discovers New Moons Orbiting Saturn (July 28, 1995).

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